Polyolefin resin expanded molded products are widely used in applications of shock-absorbing packing materials, returnable containers, materials for automobiles (e.g., toolboxes, floor core materials, etc.), and the like. However, these polyolefin resin expanded molded products sometimes generate annoying high-frequency frictional sound (squeaky sound) when rubbed against each other or against some other plastic product or metal product or the like.
Known methods for preventing frictional sound from a polyolefin resin expanded molded product are: a method of obtaining a polyolefin resin expanded molded product by molding expanded polyolefin resin particles which are obtained by expanding resin particles composed of a resin composition obtained by melting and kneading a polysiloxane into a polyolefin resin (Patent Literature 1); a method of obtaining a polypropylene resin expanded molded product by molding expanded polypropylene resin particles composed of a resin composition which is obtained by melting and kneading a polyethylene wax having a molecular weight of 1000 or greater and 4000 or less into a polypropylene resin (Patent Literature 2); and the like.
However, in these methods, there is room for improvement in the ability to continuously prevent the frictional sound. Furthermore, in the method of Patent Literature 1, the polysiloxane bleeds out of the base material resin and thus there is room for improvement in the fusibility of expanded particles when the expanded molded product is produced. In the meantime, the method of Patent Literature 1 utilizes a lipophilic hydrocarbon (e.g., butane) as a foaming agent when producing expanded particles.
In this regard, the inventors conducted a study on changing the foaming agent to water and/or carbon dioxide gas in the method of Patent Literature 1, and the study revealed that the use of water and/or carbon dioxide gas as a foaming agent reduces the foamability during the production of expanded particles. The inventors conducted a subsequent study and inferred the following. In the case where water and/or inorganic gas is/are used as a foaming agent, it is necessary to increase the moisture content of resin particles by allowing the resin particles to absorb water to increase foamability. However, the water repelling effect of the siloxane inhibits the water absorption by the resin particles. This results in a reduction in foamability.
Patent Literatures 3 and 4 are also examples of literatures that disclose techniques related to expanded polypropylene resin particles and polypropylene resin compositions.